1. Field of the Invention
The present invention relates to an electronic device having electric wires such as a wire-wound chip coil and to a method of producing such a coil device. The present invention also relates to a method of producing an electronic device having electric wires in an inductance component. More particularly, the present invention relates to a method of producing an electronic device having electric wires including an improved process of connecting end portions of an insulated electric wire wound around a core to electrodes located on the core.
2. Description of the Related Art
FIG. 5 illustrates a conventional chip coil made up of an electric wire 5 wound around a core 1 made of a magnetic material, wherein ends 5a of the wire 5 are connected, via thermo-compression bonding such as a wire bonding process, to respective electrodes 2 located on the core 1. The electrodes 2 are made of a material such as Ag or Ag--Pd. When the ends 5a of the wire are connected to the respective electrodes 2, the resultant connecting portions of the wire 5 have raised portions bulging from the surface of the electrodes 2. The bulging shape of the connecting portions causes the chip coil to become unstable when it is mounted on a printed circuit board. That is, the chip coil is mounted unevenly in a slanted orientation or topples over and is separated from the printed circuit board in the worst case. Another problem with the chip coil of this type is that the ends 5a of the wire 5 are exposed directly to air and thus, the ends of the wire are oxidized. This makes it difficult to solder the ends of the wire during the process of mounting the chip coil.
One possible technique of improving the stability of the mounted position is to form recesses 3 as seen in FIG. 6 in both end portions of the core 1 so that the ends 5a of the wire can be placed inside the recesses 3. However, the shape of the core 1 becomes complicated and difficult processes are required to produce such a complicated structure including the recesses 3.
The insulated electric wire 5 generally consists of an electric wire made of metal such as copper whose outer surface is coated with an insulating material such as polyesterimide. The insulated electric wire is connected to the electrodes via, for example, pulse heating. The connection process via pulse heating is described below with reference to FIG. 11.
In FIG. 11, cross sections of the core 51 of the coil device and the electrode 52 located on the upper surface of the core 51 are shown. An end portion of the insulated electric wire 53 is disposed on the electrode 52.
A pressing tip 56 heated at about 500.degree. C. is moved down so that the insulated electric wire 53 is pressed against the electrode 52. The electric wire 53a is flattened by the pressure and the electric wire 53a is connected to the electrode 52 via thermo-compression bonding.
In this connection technique, if the electrode 52 is made of metal having a high melting point such as Ag, Cu, or Ni, the insulating coating 53b melts at a temperature lower than the melting point of the electrode 52, and the electric wire 53a and the electrode 52 are directly connected to each other. Another feature of this technique is that the electric wire 53a is flattened by the pressure.
However, although the electric wire 53a on the electrode 52 is flattened, there is still a processing step required on the surface of the electrode 52 and the electric wire 53a. When the coil device is mounted on a printed circuit board such that the surface of the electrode 52 and attached electric wire 53a comes into contact with the printed circuit board, the above-described step can cause the coil device to become unstable or cause the soldered connection to become unreliable.
In many cases, the surface of the electrodes 52 of the coil device is plated with metal having a low melting point such as Sn or solder so that a low-melting-point electrode layer is formed on the electrode 52 thereby ensuring that the electrode can be easily soldered. For example, as shown in FIG. 12, the electrode 52 is produced by coating a silver-filled paste on the surface of the core 51 and baking it so as to form a base layer 52a, then plating the surface of the base layer 52a with Ni thereby forming a Ni-plated layer 52b for protecting the base layer 52a from being eroded by solder, and finally forming an electrode layer 52c of low-melting-point metal which allows the electrode 52 to be easily soldered.
In the case of the coil device having the above structure, when connection is performed with the pressing tip 54 heated at about 500.degree. C., the electrode layer 52c is heated by the pressing tip 54 to a temperature higher than the melting point of the low-melting-point metal. In the connecting process, the electrically conductive wire 53a is pressed by a pressure high enough to compress the electrically conductive wire 53a. As a result, the insulating coating 53b and the electrode layer 52c made of the low-melting-point metal at the top layer are both melted into liquid states, and the electrically conductive wire 53a is compressed into a flattened shape.
As a result, in the pressing process using the pressing tip 54, the low-melting-point metal in the liquid state is pushed aside by the insulating coating 53b in the liquid state toward the sides of the electrically conductive wire 53a. In the above process, after the low-melting-point metal is pushed aside by the melted insulating coating 53b, if the melted insulating coating 53b sticks to the pressing tip 54 and is removed when the pressing tip 54 is moved up to its original position, the Ni-plated layer 52b is sometimes exposed in an area A at a side of the conductive wire 53a. If the Ni-plated layer 52b is partially exposed, when the coil device is mounted on a printed circuit board via soldering, a connection failure can occur because the Ni-plated layer 52b has poor solder wettability.
Even in the case where the Ni-plated layer 52b does not become exposed in the area A in FIG. 12 after the low-melting-point metal is pushed aside by the melted insulating coating 53b, if the melted insulating coating 53b sticks to the pressing tip 54 and is removed when the pressing tip 54 is moved up to its original position, a crater 52d is produced in an area on the surface of the electrode 52 where the insulating coating 53b of the electrically conductive wire 53a was present. If such a crater is produced, it becomes difficult to make a good connection in the soldering process.